In recent years, attempts have been made to elucidate functions of synthetic peptides, proteins, DNAs, and sugars or induce special functions by introducing these substances into a cell to adjust intracellular protein interaction and control intracellular signaling, transcription, or the like. With such an approach, it can be expected that genetic information that has been considered to be a mystery is elucidated, the causes of diseases are elucidated, and methods for treating the diseases are developed. Moreover, due to development of ES cells and iPS cells, a technique for controlling cell functions with nucleic acids or proteins becomes more important.
Water-soluble polymer substances such as polypeptides, nucleic acids, and sugars are usually highly hydrophilic, and thus have difficulty in passing through a cell membrane. Therefore, a microinjection method, an electroporation method, a calcium phosphate method, a lipofection method, a viral vector method, a membrane-permeable peptide method, and the like are known as a method for introducing these substances into a cell.
Of these methods, the membrane-permeable peptide method is a method that utilizes macropinocytosis of a cell induced by a membrane-permeable peptide. A method in which a target compound to be introduced into a cell and a membrane-permeable peptide are covalently bound and introduced (see Patent Literature 1 and 2, for example), and a method in which a polymer compound having a membrane-permeable peptide in a side chain and a target compound to be introduced into a cell are allowed to coexist and only the target compound is introduced (see Patent Literature 3 and 4, for example) are known as the membrane-permeable peptide method. With the method in which a target compound and a membrane-permeable peptide are covalently bound and introduced, cells are less damaged, but complex pretreatment is needed. On the other hand, the method using a polymer compound having a membrane-permeable peptide in a side chain is simple, but when a conventionally known polymer compound having a membrane-permeable peptide in a side chain is used, a water-soluble polymer substance is introduced with an insufficient efficiency, and a concentration at which the polymer compound is used is sometimes limited due to weak cytotoxicity of the polymer compound. Therefore, there is a problem with such a polymer compound when the efficiency of introducing the target compound is improved.
Also, the application of the polymer having a membrane-permeable peptide in a side chain to an agent for promoting the absorption of a medicine from the epithelium (see Patent Literature 5, for example) is proposed, but it is necessary to use the polymer at a high concentration in order to obtain a sufficient absorption promoting effect, and thus there is a concern about the irritation of a mucous membrane when the agent is put into practical use.